In a conventional application where the farmer employs practices which leave little or no residue on the surface, dry fertilizer may be broadcast on the surface of the field, and a tillage implement, such as a chisel plow or field cultivator, may be used to mix the fertilizer with the top several inches of the soil being tilled.
Where no-till (or "minimum-till" or "high residue") farming practices are employed by the farmer, surface residue is present, and it is desired to leave the surface soil undisturbed as much as possible. Row crop planters designed for use in high residue conditions may have one to three coulters mounted in front of each planter row unit to perform a minimum amount of tillage ahead of the actual planting unit. This is insufficient for widespread mixing of soil and fertilizer, so surface application of fertilizer is impractical in combination with current planting practices. Chisel plows have also been suggested for tillage in high residue conditions. Some such chisel plows are designed to minimize the disturbance of the soil at the surface. Neither of these techniques just described have been widely used for the simultaneous application of fertilizer and deep tillage under high residue conditions.
Currently, there are a number of methods for applying fertilizer in high residue conditions. One such method uses very thin applicator knives which part the soil. A tube is located behind the knife to deposit the fertilizer in the slot formed by the knife. The disadvantage of this system is that the knife wears thin, and the deposit tube wears on the sides as they scrape against the walls of the slot.
Another method employs a coulter having ripples in the surface of the coulter extending radially outwardly of its center. The slot formed in this method is comparatively narrow so that the side walls of the slot are vertical and adjacent one another leaving a thin slot of uniform width. In the case of applying liquid fertilizer, because of the somewhat dry soil conditions in which fertilizer applicators of this type are used, the liquid fertilizer sometimes contacts the side walls of the slot and is "wicked" into the soil too close to the surface for effective application. Deeper fertilizer placement is more desirable.
Another method of applying liquid and dry fertilizer employs a flat coulter which is inclined in a vertical plane offset approximately seven degrees from the direction of travel--that is, the yaw of the plane of the disc is approximately seven degrees. These devices are generally used on row crop planters which are, of course, not designed to operate at speeds much higher than 4-6 miles an hour, and it is difficult to deposit the fertilizer at a depth greater than approximately two inches in devices of this nature because the coulter encounters high stress at depths deeper than two inches due to its substantial inclination relative to the line of travel.